I've been to OSH 8 of the last 10 years and have yet to have a problem getting a camping spot. It might be full for the first part of the first day, but there are a lot of pre-OSH arrivals that actually leave during the first day.
Just keep checking the website and/or the 1-800 number and try your luck, I guess.
FYI: Seems like there is always a large influx of planes immediately following the airshow, but getting there EARLY AM (as in listening to the controllers sip their coffee on the radio) usually works for us.